1. Field of the Invention
The present invention relates to an FM receiver and a pilot detector thereof, and a method for determining a type of a processed signal; in particular, relates to an FM receiver and a pilot detector thereof, and a method for determining whether a processed signal is stereo or mono.
2. Descriptions of the Related Art
Frequency modulation (FM) signals are frequently used for the purpose of wireless communications. Each FM signal carries a pilot signal at a certain frequency, e.g., 19 kHz to indicate that an audio signal demodulated from the FM signal is stereo or mono.
FIG. 1 shows a FM receiver 1 which comprises a selection circuit 101, a high frequency (HF) amplifier 103, a mixer 105, an intermediate frequency (IF) amplifier 107, an FM demodulator 109, a stereo decoder 111, a loudspeaker 113, and an antenna 115. There are many radio waves at different frequencies in the air, and the selection circuit 101 is configured to select a preferred channel and receive an HF signal in the preferred channel from the antenna 115. Generally speaking, the HF signal is too weak after a long distance transmission to be demodulated correctly. The HF amplifier 103 is configured to amplify the weak HF signal. Then the frequency of the HF signal is mixed by the mixer 105. The intermediate frequency of the HF signal is amplified by the IF amplifier 107. The HF signal is down converted to be an IF signal thereby. The FM demodulator 109 demodulates the IF signal to generate a multiplexed signal. Then the stereo decoder 111 analyzes the type of the multiplexed signal and decodes the multiplexed signal into a mono audio signal or a stereo audio signal according to its type. The loudspeaker 113 is configured to play sound in response to the audio signal.
To analyze the type of the multiplexed signal, the stereo detector 111 needs a pilot detector. FIG. 2 shows an operating environment of a conventional pilot detector 23. A multiplexed signal 202, such as the multiplexed signal generated from the FM demodulator 109, is filtered by a band pass filter (BPF) 21. The band pass filter 21 has a center frequency of 19 kHz to retrieve a pilot signal 204 out from the multiplexed signal 202. The pilot detector 23 comprises a hysteresis comparator 201. The pilot signal 204 is transmitted to the hysteresis comparator 201 to compare with a reference 206. With hysteresis, noise can be removed during the comparison. If a peak value of the pilot signal 204 is larger than or equal to the reference 206, an indication signal 208 of the hysteresis comparator 201 goes to, for example, a high level in order to notify the stereo decoder 111 that the multiplexed signal 202 is stereo. Otherwise, the indication signal 208 goes to a low level in order to notify the stereo decoder 111 that the multiplexed signal 202 is mono.
However, if the filter quality of the band pass filter 21 is not good or the noise is too serious, the pilot signal 204 will not be retrieved clearly enough for the pilot detector 23 to correctly determine the type of the multiplexed signal 202. In other words, the stereo decoder 111 might utilize an inappropriate decoding process to decode the multiplexed signal 202 so that the audio signal cannot be converted to distinguishable sound.
To solve the problem, a pilot detector which may determine a type of a signal correctly is needed in the industrial field.